The present invention relates to a liquid-development full-color electrophotographic apparatus, and more particularly, to a liquid-development full-color electrophotographic apparatus which optimally controls transfer of toner images in a plurality of colors from a photosensitive drum(s) to an intermediate transfer roller.
As an electrophotographic apparatus operating by the steps of generating an electrostatic latent image on a photosensitive body (a photosensitive drum), causing toner to be attracted to the electrostatic latent image, transferring the toner onto paper or the like, and fixing the transferred toner, a dry-type apparatus, which uses a powder toner, is widely used.
However, a powder toner involves the following problems: toner particles scatter; and since toner particles have a relatively large particle size of 7 xcexcm to 10 xcexcm, resolution is low.
Thus, when high resolution is required, a liquid-development-type apparatus, which uses a liquid toner, is used for the following reason. A liquid toner has a small toner particle size of about 1 xcexcm and exhibits a large electrostatic-charge capacity. Thus, a toner image is unlikely to be disturbed, and high resolution can be achieved.
FIG. 8 shows the overall configuration of a conventional liquid-development-type electrophotographic apparatus (disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 2000-056575). In FIG. 8, a photosensitive drum 10 is electrostatically charged by means of a charger 21. Subsequently, the photosensitive drum 10 is exposed to light by means of an exposure unit 22, whereby an electrostatic latent image is formed. A prewetting unit 23 applies, for example, silicone oil to the surface of the photosensitive drum 10.
Developing units 24 corresponding to yellow, magenta, cyan, and black are provided and use as a liquid developer a nonvolatile, high-viscosity, high-concentration liquid toner. A developing roller supplies the liquid developer onto the photosensitive drum 10 while causing toner particles contained in the liquid developer to adhere to the photosensitive drum 10 according to an electric field established between the same and the photosensitive drum 10.
An intermediate transfer roller 15 transfers color toners one by one from the photosensitive drum 10 according to an electric field established between the same and the photosensitive drum 10. When the intermediate transfer roller 15 transfers toner particles from the photosensitive drum 10 according to an electric field established between the same and the photosensitive drum 10, oil which is composed of excessive prewetting liquid and carrier in a developed toner layer and which, together with toner particles, is transferred from the photosensitive drum 10 to the intermediate transfer roller 15. In order to remove the oil, the intermediate transfer roller 15 is equipped with an oil-removing roller 25.
A heating unit 28 heats the surface of the intermediate transfer roller 15 to thereby melt toner adhering to the intermediate transfer roller 15. Heating by the heating unit 28 is performed after all color toners have been transferred onto the intermediate transfer roller 15. A pressure roller 19 is adapted to fix on a printing medium the toners which are melted on the intermediate transfer roller 15 by means of the heating unit 28. Reference numeral 26 denotes a blade for scraping off residual development toner, and reference numeral 27 denotes a destaticizer.
In such a single-photosensitive-drum-type electrophotographic apparatus, which uses a single photosensitive drum 10, in order to transfer toner images in four colors from the photosensitive drum 10, the intermediate transfer roller 15 must be rotated four rotations. This configuration is disadvantageous in terms of printing speed.
High-speed printing can be implemented through employment of four photosensitive drums corresponding to four colors. Toner images formed on the corresponding photosensitive drums are sequentially superposed on an intermediate transfer roller. This configuration reduces the size of the apparatus. However, in a multiple-photosensitive-drum-type full-color electrophotographic apparatus, when a toner image on the surface of a photosensitive drum is to be transferred onto the intermediate transfer roller, as shown in FIG. 6, a bias voltage must be applied to an intermediate transfer roller 15.
In FIG. 6, while photosensitive drums 11-14 corresponding to four colors are grounded, a constant bias potential of, for example, xe2x88x92500 V is applied to the intermediate transfer roller 15. The bias potential causes toner images on the corresponding photosensitive drums 11-14 to be transferred onto the intermediate transfer roller 15.
However, since the color toners differ in electric characteristics depending on pigment to be used, application of a common electric potential among the colors as illustrated fails to yield an optimum transfer efficiency.
A carrier solvent to be used in liquid development is intended to prevent scattering of toner particles, which assume a particle size of about 1 xcexcm, as well as to uniformly disperse toner particles through electrification of the toner particles. In development and electrostatic transfer processes, the carrier solvent serves as a xe2x80x9cbridgexe2x80x9d to facilitate movement of toner particles, which is effected by means of electric-field action.
When color toners are transferred one by one from a photosensitive drum to an intermediate transfer roller, all toner particles are transferred according to an electric field established between the photosensitive drum and the intermediate drum. However, in actuality, some toner particles which have previously been transferred onto the intermediate transfer body may be reversely transferred onto the photosensitive drum.
FIG. 7 is a view for explaining reverse transfer from the intermediate transfer body to the photosensitive drum. The illustration shows a state in which a second color toner is transferred onto the intermediate transfer body onto which a first color toner has already been transferred, to thereby super pose the second color toner on the first color toner. Essentially, all toner particles are expected to be transferred and superposed on the intermediate transfer body as a result of being subjected to an electric-field action. However, in some cases, some of the toner particles of the first color are reversely transferred onto the photosensitive drum. This is considered undesirable.
An object of the present invention is to provide a liquid-development full-color electrophotographic apparatus wherein, in order to optimally control transfer of toner images in a plurality of colors from a photosensitive drum(s) to an intermediate transfer roller, an optimal transfer bias is applied for each of the colors according to electrical characteristics of the corresponding color toner so as to yield optimal transfer efficiency for the color toner.
Another object of the present invention is to provide a liquid-development full-color electrophotographic apparatus wherein, in order to optimally control transfer of toner images in a plurality of colors from a photosensitive drum(s) to an intermediate transfer roller, reverse transfer of a toner image from the intermediate transfer roller to a photosensitive body is prevented in the course of superposing transfer, to thereby prevent image deterioration.
A liquid-development full-color electrophotographic apparatus of the present invention comprises a development section using a liquid toner as a liquid developer, the development section being in contact with a photosensitive drum, on which an electrostatic latent image is formed, so as to supply the liquid developer onto the photosensitive drum, and causing toner particles contained in the liquid developer to adhere to the photosensitive drum according to an electric field established between the development section and the photosensitive drum to thereby form a toner image; an intermediate transfer roller to which the toner image is transferred from the photosensitive drum according to an electric field established between the same and the photosensitive drum; and a transfer-and-fixation section for further transferring the toner image from the intermediate transfer roller onto an intermediate transfer belt and then melting the toner image through application of heat at a contact portion between the intermediate transfer belt and a printing medium to thereby melt-transfer the toner image onto the printing medium. A plurality of photosensitive drums are provided in such a manner as to correspond to liquid toners in a plurality of colors. Toner images formed on the photosensitive drums corresponding to the respective colors are sequentially transferred and superposed on the intermediate transfer roller. An electric field for transferring a toner image from each of the photosensitive drums to the intermediate transfer roller is established through application, to the corresponding photosensitive drums, of a voltage controlled according to electric characteristics peculiar to a color toner on the photosensitive drums while the intermediate transfer roller is grounded.
A liquid-development full-color electrophotographic apparatus of the present invention comprises a development section using liquid toners in a plurality of colors as liquid developers, the development section being in contact with an image bearer body, on which an electrostatic latent image is formed, so as to supply the liquid developer onto the image bearer body, and causing toner particles contained in the liquid developer to adhere to the image bearer body according to an electric field established between the development section and the image bearer body to thereby form a toner image in a corresponding color; and an intermediate transfer body to which the toner image for each of the plurality of colors is transferred from the image bearer body, the transferred toner images being superposed on one another. The liquid-development full-color electrophotographic apparatus further comprises means for enhancing the degree of toner cohesion of a toner image transferred onto the intermediate transfer body in order to suppress reverse transfer of a previously transferred toner image to the image bearer body; and a transfer-and-fixation section for melting a toner image formed, through transfer and superposition, on the intermediate transfer body through application of heat at a contact portion between the intermediate transfer body and a printing medium to thereby melt-transfer the toner image onto the printing medium.